This invention relates to the field of conductors and methods of manufacturing conductors, especially with regards to lightweight composite electrical wires.
In many applications, the weight of an electrical conductor is not an issue. Rather, cost, conductivity, flexibility and longevity are primary considerations. Copper has been the obvious choice for many applications because of its availability, ductility, moderately low cost, and high conductivity.
During the 20th century, alternative conductors were developed for special applications. Examples include semiconductors for integrated circuits, superconductors for powerful electromagnets, and aluminum for utility power transmission. Aluminum wire also had brief widespread use for general wiring. However, this ended following numerous house fires that were attributed to inadequately designed electrical terminations. Interestingly, there is little evidence of fire caused by the copper-clad variety of aluminum wire, which is still in use in many installations. Nonetheless, designers are presently reluctant to use any non-cuprous wire that is not definitely proven to be safe, and rightly so. The vast majority of electrical wiring remains copper or some alloy thereof, even in aerospace applications where a premium is gladly paid to reduce weight.
It is commonly believed that copper and silver are the “best” room temperature conductors. However, many fail to realize the arbitrary historical basis of this supposed superiority. Conductivity has conventionally been defined as a measure of the ability of a unit volume of a material to conduct electricity. Another useful definition, however, would characterize the ability of a unit mass to conduct electricity. When conductivity was first defined, the distinction between the volume and mass of a conductor was irrelevant because it was rarely important to reduce the weight of electrical wire. Over the years, however, machines have become increasingly mobile, energy prices have increased, and reduction of weight has assumed far greater importance.
Density adjusted conductivity is a measure of conductivity per unit mass, and is easily calculated by dividing conventional conductivity by mass density. Density adjusted conductivity is a more useful figure of merit for comparing different types of conductors for possible use in applications where electrical conduction with minimal weight is desired.
Sodium, for example, has roughly one-third the conductivity of copper but approximately one-ninth the density. Thus, the density adjusted conductivity of sodium is approximately three times that of copper. Were one to replace a copper wire with an equivalent length of sodium wire having three times the cross-sectional area, the thicker sodium wire would have the same conductance as the copper, but would weigh only one-third as much. Hence, per unit mass, sodium conducts constant current at least three times better than copper.
A century ago, most conductors carried constant current for long periods of time through insulators with low maximum service temperature, primarily for such uses as electrical lighting and motors. Today, however, many conductors carry brief pulses of electricity separated by relatively long idle periods. Also, modern insulators are often capable of withstanding very high temperatures. An extremely lightweight wire that tolerates intense current, if only for a brief period of time, is of much greater usefulness today than a century ago.
Most wire continues to be sized on the traditional basis of continuous operation, where heat generation from electrical resistance is in thermodynamic equilibrium with the rate of heat rejection from the wire into the environment. However, today most electrical conductors operate in thermal disequilibrium. During a brief pulse, the wire is heated much faster than it is cooled. Later, when no longer conducting electricity, most of the resistive heat from the wire is released to the environment. A wire's “impulse tolerance” (number of ampere seconds of brief impulse a unit mass of wire can repeatedly tolerate) is often a more useful measure of a wire's suitability to an application than the number of amperes the wire would tolerate if operated continuously.
Based on the more useful criteria of density adjusted conductivity and impulse tolerance, which conductive elements are best? Surprisingly, not silver and copper but rather the lightweight alkali metals sodium and lithium, which both have more than three times the density adjusted conductivity as copper, and, under common conditions, on the order of one thousand times the impulse tolerance.
Factors Affecting Impulse Tolerance
The impulse tolerance of non-superconducting wire is largely dependent on a wire's ability to tolerate resistive heat produced during an impulse, which is a composite function of electro-thermodynamic performance at each of several stages that largely occur in chronological succession: resistive heat production in solid metal, temperature increase, possible melting, continued resistive heat production in liquid metal, cessation of impulse, transfer of heat to the environment, possible refreezing, and cooling back to ambient temperature. Performance at each stage depends on different material properties. The best conductor would perform well during all of these stages.
For a given amount of current, the heat produced per unit mass is inversely proportional to the density adjusted conductivity of the solid metal. During a brief impulse, almost all of the heat that is produced stays in a wire, increasing its temperature. Increased temperature decreases conductivity, and if high enough, damages a wire or adjacent components. The temperature increase of a wire per unit mass per given amount of resistive heating is inversely proportional to the wire's specific heat. If the temperature of a wire is high enough to melt the wire, then heat is absorbed by the process of melting.
The amount of heat absorbed during melting is proportional to the wire's heat of fusion. Once melted, the rate of resistive heat production is inversely proportional to the density adjusted conductivity of the liquid metal.
After an impulse is finished, the rate of heat rejection per unit mass depends on several parameters.
Why Sodium and Lithium Wire have Superior Impulse Tolerance
At every step of the process of impulse conduction, the material properties of lithium and sodium cause them to outperform all other metal elements. As discussed above, sodium and lithium have the highest density adjusted conductivity of all the elements. Thus, they generate the least amount of heat per unit mass when conducting electricity. They have very high specific heat, so the resistive heat that is produced increases the temperature of the metals relatively little.
Conductors are usually thought of as solid material. Melting of a conductor is commonly considered synonymous with structural failure, and occurring at an unacceptably high temperature is likely to cause fire. However, sodium's and lithium's surprisingly low melting points (97.7° C. and 180.5° C., respectively) are entirely compatible with maximum service temperatures present in many applications. Not only is melting thermally tolerable by most adjacent components, it is surprisingly advantageous, as both metals have a high heat of fusion that provides absorption of a tremendous amount of heat per unit mass during melting.
Both sodium and lithium have high volume per unit mass, thus high surface area per unit mass, which aids heat transfer. If melted, heat transfer is further aided by the temperature of the wire not falling below the melting point during most of the cooling process as the molten metal refreezes. The temperature difference between the wire and its environment is thereby held at a relatively high level throughout most of the cooling process. By adding a small amount of lithium to sodium, the melting point may be increased to just under the maximum service temperature of surrounding components, maximizing the temperature gradient and resultant heat transfer. Compared to denser, continuously solid conductors whose rate of cooling immediately starts decreasing with decreasing temperature during cooling, permissibly fusible sodium and lithium conductors lose heat more quickly, thereby tolerating greater and more frequent impulses. Both sodium and lithium lose some conductivity as they melt, as do all metals. However, even when completely melted, sodium and lithium continue to have surprisingly high mass adjusted conductivity. Even at 200° C., the density adjusted conductivity of sodium is surprisingly still better than 200° C. solid copper. (200° C. molten lithium has density adjusted conductivity only slightly worse than 200° C. solid copper).
The low melting point, high heat of fusion, low mass density and comparatively small increase in heat-producing resistivity when melted combine to prevent the temperature of a permissibly fusible sodium or lithium conductor from ever exceeding its melting point under a wide range of operational conditions. This represents a surprising benefit of “intrinsic thermal control” not usually associated with conductive metals.
Copper for example has little intrinsic thermal control. Due to its low specific heat, the temperature of a piece of copper wire increases rapidly in response to resistive heating. In fact, this effect is so pronounced that the size of most copper wire is chosen primarily by the maximum permissible wire temperature rather than the optimal trade-off between wire mass and resistive energy loss. Potential economies from more resistive but lighter copper wire are lost because the wire temperature would unsafely exceed thermal limits.
To maximize heat flow out of an alkali metal wire, the melting point may be adjusted to just under the maximum service temperature of adjacent components by choosing an alkali metal alloy that has the desired melting point. A designer may then choose the size of an alkali metal alloy conductor by balancing the weight penalty of a larger wire with the electrical cost penalty from the increased resistance of a smaller wire. The minimum size possible is that which is just sufficient to tolerate the maximum expected impulse by completely melting and rising to the maximum service temperature of adjacent components. A copper wire of the same weight, when exposed to the same impulse, would not only generate more than three times the total heat, but would also exhibit a spike in temperature that would greatly exceed the thermal tolerance of surrounding components.
Electro-thermodynamic calculations show a sodium or lithium conductor designed to completely melt can handle on the order of 1000 times the electrical impulse as a non-meltable copper wire of equivalent mass heated to the same temperature. Thus, a copper wire designed to barely tolerate a given magnitude of impulse can potentially be replaced with a fusible lithium or sodium wire weighing only one thousandth as much.
Sodium in particular has numerous characteristics that recommend its use as an electrical conductor. Made from ordinary salt, it is limitlessly available and very inexpensive, especially compared to copper. It has heat of fusion second only to lithium, excellent intrinsic thermal control at a convenient melting point of 97.7° C., retains about half its conductivity when melted, cools quickly due to high surface to mass ratio, has impulse tolerance second only to lithium and has the highest density adjusted conductivity at room temperature of any material known to man.
Problems with Alkali Metal Conductors
The excellent electro-thermodynamic properties of lithium and sodium, however, come with a number of very inconvenient chemical and physical properties that have heretofore made them impractical for widespread use. They react strongly with almost all materials when heated, especially when melted. They ignite easily not only in oxygen, but most other common gaseous environments. Sodium burns just below its 883° C. boiling point, emitting caustic fumes onto surrounding structures. If sodium or lithium is doused with water, hot explosive hydrogen gas is generated. In fact, all common fire extinguishing agents actually exacerbate alkali metal fires.
Neither sodium nor lithium is pyrophoric, i.e. exposure of solid sodium or lithium to air does not spontaneously produce fire. However, they do rapidly oxidize into caustic and nonconductive material that may corrode adjacent components.
Because alkali metals are extremely reactive and have low tensile strength, they are of no practical use unless encased in a protective and reinforcing casing. Containment is complicated by alkali metals' extremely high coefficient of thermal expansion. Sodium in particular has the highest thermal coefficient of expansion of all metallic elements. Refractory containment materials are generally heavy, have comparatively low coefficients of thermal expansion and have limited elastic range which limits bendability. Flexible polymer containment may allow wire thinning from stretching that produces hot spots from increased current density. Water vapor penetrating through polymer produces destructive hydrogen gas and sodium hydroxide. Most importantly, containment with polymer adds weight without directly adding any electrical conductance, unlike the case of metallic containment where weight penalty is partially overcome by electrical conduction through the container's wall.
Inventors have been trying for more than a century to enjoy various electro-thermodynamic and economic advantages of alkali metal conductors without suffering the chemical and physical problems described above. Limited success in a few circumscribed applications has heretofore not extended to widespread commercial acceptance for a variety of practical reasons, including but not limited to: flammability, lack of a practical means of safe wire termination, excessive weight, unreliable protection from external reactive environment, prohibitively expensive means of manufacture, inadequate flexibility, and lack of means to easily cut the wire to any desired length without special tools or knowledge.
It is the purpose of the present invention to provide a practical conductor that exploits the potential advantages of alkali metal conductors while overcoming the limitations of prior art to provide a lightweight, safe, reliable, flexible, easily connected, electro-thermo-dynamically superior, easily customizable and less expensive alternative to copper wire suitable for most wiring applications.